In order to treat water in aqueous systems with scale inhibitor, it is necessary to monitor the levels of organic phosphonate in the water so that the dosage of scale inhibitor necessary to inhibit phosphate-based scale, principally calcium phosphate, may be determined. The process presently used in the industry involves boiling the sample of water in the presence of acid and persulfate or digestion by ultraviolet light radiation in the presence of persulfate in order to convert organic phosphorus compounds to orthophosphate. The method of the instant invention does not require boiling or the use of special laboratory equipment for the conversion step.